Known are radar measuring devices for determining the fill level of a fill substance in a container work according to the travel-time difference method. Travel time difference methods utilize the physical law, according to which the travel distance equals the product of travel time and propagation velocity. In the case of fill level measurement, the travel distance corresponds to twice the separation between antenna and surface of the fill substance. The echo signal, thus the signal reflected on the surface of the fill substance, and its travel time, are determined based on the so-called echo function, respectively the digitized envelope curve, respectively the intermediate frequency curve, wherein these show the amplitudes of the echo signals as a function of the separation ‘antenna —surface of the fill substance’. The fill level can then be ascertained from the difference between the known separation of the antenna from the floor of the container and the separation of the surface of the fill substance from the antenna determined by the measuring. Microwave measuring devices using pulse radar are sold by the assignee under the mark MICROPILOT®.
Known from European Patent, EP 2045617 B1 (U.S. Pat. No. 7,633,434) is a pulse radar device for determining or monitoring the fill level of a fill substance in a container. A transmission clock generator produces a transmission clock signal having a predetermined transmission clock frequency. A transmission pulse generator, which is triggered by the transmission clock signal, forwards transmission pulses with the transmission clock frequency to an antenna. The antenna radiates the transmission pulses in the direction of the surface of the fill substance and receives the echo signals reflected on the surface of the fill substance. The echo signals are sampled with a sampling clock frequency, which differs slightly from the transmission clock frequency. A sampling pulse generator, which is triggered by the sampling clock signal, produces the sampling pulses with the sampling clock frequency. For the purpose of producing an intermediate signal, the echo pulses are mixed with the transmission pulses in a mixer. Via an integrator, the intermediate frequency signal is integrated. The integrator is a sample/hold circuit, which has a switch and a capacitor. The switch is so designed that the intermediate frequency signal is sampled with the sampling clock frequency. This known circuit arrangement is scarcely implementable on a semiconductor chip.